This invention relates to ink jet systems and, more particularly, to a new and improved ink jet apparatus for use with hot melt inks providing controlled solidification of such inks.
Ink jet systems using inks prepared with water or other vaporizable solvents require drying of the ink (i.e., vaporization of the solvent) after it has been applied to a substrate, such as paper, which is supported by a platen. To facilitate drying of solvent-based inks, heated platens have previously been provided in ink jet apparatus.
Certain types of ink jet apparatus use inks, called "hot melt" inks, which contain no solvent and are solid at room temperature, are liquefied by heating for jet application to the substrate, and are resolidified by freezing on the substrate after application. In addition, the application of hot melt ink to a substrate by an ink jet apparatus transfers heat to the substrate. Moreover, the solidification of hot melt ink releases further thermal energy which is transferred to the substrate and supporting platen, which does not occur with the application of solvent-based inks. With high-density coverage this can raise the temperature of the paper and the platen above limits for acceptable ink penetration.
In order to control the penetration of hot melt inks into a permeable substrate such as paper to the desired extent, it is advantageous to preheat the substrate to a temperature close to but below the melting point of the hot melt ink. If the substrate temperature is too cold, the ink freezes after a short distance of penetration. This results in raised droplets and images with an embossed characteristic. Additionally, such ink droplets or images may have poor adhesion or may easily be scraped off or flake off by action of folding or creasing or may be subject to smearing or offsetting to other sheets. If the paper temperature is too high, for example, higher than the melting point of the ink, the ink does not solidify before it has penetrated completely through the paper, resulting in a defective condition called "print-through". In addition, an image printed on a substrate which is at a temperature in the vicinity of the melting point of the hot melt ink will appear noticeably different than an image printed at a lower substrate temperature. Such images exhibit characteristics of larger-than-normal spot size, fuzzy edges, blooming of fine lines and the like. Furthermore, contrary to the conditions required for the use of solvent-based inks in an ink jet apparatus, heating of the substrate after the ink has been deposited is ineffective to control the spread of the drops and to prevent the above-mentioned difficulties which may occur when using hot melt inks. Consequently, presently known ink jet apparatus using unheated or even heated-only platens are incapable of maintaining the conditions required for effective application of hot melt ink to a substrate to produce constant high-quality images.